This invention deals generally with farm machinery and more specifically with an electronic speed control for self propelled farm machines.
It is quite interesting that large self propelled farm machines are not all powered or steered in the same manner as the road vehicles with which we are familiar. Such machines are not powered by direct mechanical linkages from the engine to the drive wheels, and are not steered by changing the angle of the vertical planes of the wheels. The wheels of many such self propelled farm machines, including large farm tractors, are coupled to their engines by hydraulic systems. The engine directly drives hydraulic pumps, and those pumps are connected by hoses to hydraulic motors that are located at and turn the wheels. Wheel speed is then controlled by changing the quantity of hydraulic fluid that the pump delivers to the wheel motor. Furthermore, to reverse the motion of the hydraulic motor and thus reverse the direction of the wheel the direction of the hydraulic fluid flow is reversed.
It is even more interesting to note that the steering of such self propelled farm machines is accomplished by the very same system as the speed and forward and reverse directional control. The very large wheels of such equipment are not steered as automobile wheels are, but instead, the direction of the machine is changed by driving the wheels at different rotational speeds. Thus, for a typical self propelled farm machine with two forward drive wheels and two caster type smaller rear wheels, if the right forward wheel is stopped and the left forward wheel is rotated forward, the tractor will turn toward the right. In fact, if instead of being stopped, the right forward wheel is rotated in reverse at the same speed the left wheel is rotated forward, the tractor will turn right around the center of its own wheel axis.
Typically, the speeds of the wheels have been controlled by the machine operator using a lever with a direct mechanical linkage to a speed control rod interconnected with the two hydraulic pumps, one each for the right and the left wheels. The steering control also acts on these same two hydraulic pumps. The steering wheel is used to rotate the speed control rod that is connected to the pump interconnection linkages. The rotation of the speed control rod is around its own axis, and the pump linkages themselves are connected to the control rod by oppositely threaded collars. Thus, when the speed control rod is rotated, the pump linkages either come closer together or farther apart, depending upon the direction of rotation. This change in the spacing of the pump linkages changes their response to the motion of the speed control rod, and thus causes the right and left wheels to operate at different speeds and the farm machine to turn.
There is a significant trend in the farm equipment industry to automate farm equipment. It is particularly advantageous for very large fields to operate these machines under conditions where their speed is held closely to a setting set by the operator. In effect, it means placing self propelled farm machines in a “cruise control” mode. However, farm machines are subjected to rapidly occurring and widely varying load conditions that make any speed control difficult, and automatic speed control particularly difficult. One example is the condition of a machine suddenly coming under full load and therefore causing the engine to slow down, but instead being asked by either an automatic control system or an operator to regain speed. Such a condition is particularly likely when a machine includes an automatic speed control that quickly recognizes only that the machine is slowing down from the selected speed, and therefore automatically attempts to increase the speed. Under such conditions the engine, which is already fully loaded, will stall unless the operator intercedes and actually reduces the speed setting. With automatic speed controls becoming much more common for the large machines, it would be very desirable to have a control system that not only regulates the machine ground speed based on a control setting by the operator, but would also assure that no situation arises that causes the engine to stall because of loading.